In a normal heart, cells of the sinoatrial node (SA node) spontaneously depolarize and thereby initiate an action potential. This action potential propagates rapidly through the atria (which contract), slowly through the atrioventricular node (AV node), the atriventricular bundle (AV bundle or His bundle) and then to the ventricles, which causes ventricular contraction. This sequence of events is known as normal sinus rhythm (NSR). Thus, in a normal heart, ventricular rhythm relies on conduction of action potentials through the AV node and AV bundle.
Rhythms that do not follow the sequence of events described above are known as arrhythmias. Those that result in a heart rate slower than normal are known as bradyarrhythmias; those that result in a faster heart rate than normal are called tachyarrhythmias. Tachyarrhythmias are further classified as supraventricular tachyarrhythmias and ventricular tachyarrhythmia. Supraventricular tachyarrhythmias (SVTs) are characterized by abnormal rhythms that may arise in the atria or the atrioventricular node (AV node). For example, a paroxysmal SVT can exhibit heart rates between approximately 140 beats per minute (bpm) and approximately 250 bpm. However, the most common SVTs are typically atrial flutter (AFI) and atrial fibrillation (AF). In addition, many SVTs involve the AV node, for example, AV nodal reentry tachycardia (AVNRT) where an electrical loop or circuit includes the AV node.
Atrial flutter (AFI) can result when an early beat triggers a “circus circular current” that travels in regular cycles around the atrium, pushing the atrial rate up to approximately 250 bpm to approximately 350 bpm. The atrioventricular node between the atria and ventricles will often block one of every two beats, keeping the ventricular rate at about 125 bpm to about 175 bpm. This is the pulse rate that will be felt, even though the atria are beating more rapidly. At this pace, the ventricles will usually continue to pump blood relatively effectively for many hours or even days. A patient with underlying heart disease, however, may experience chest pain, faintness, or even heart failure as a result of the continuing increased stress on the heart muscle. In some individuals, the ventricular rate may also be slower if there is increased block of impulses in the AV node, or faster if there is little or no block.
If the cardiac impulse fails to follow a regular circuit and divides along multiple pathways, a chaos of uncoordinated beats results, producing atrial fibrillation (AF). AF commonly occurs when the atrium is enlarged (usually because of heart disease). In addition, it can occur in the absence of any apparent heart disease. In AF, the atrial rate can increase to more than 350 bpm and cause the atria to fail to pump blood effectively. Under such circumstances, the ventricular beat may also become haphazard, producing a rapid irregular pulse. Although AF may cause the heart to lose approximately 20 to 30 percent of its pumping effectiveness, the volume of blood pumped by the ventricles usually remains within the margin of safety, again because the atrioventricular node blocks out many of the chaotic beats. Hence, during AF, the ventricles may contract at a lesser rate than the atria, for example, of approximately 125 bpm to approximately 175 bpm.
Overall, SVTs are not typically immediately life threatening when compared to ventricular arrhythmias, examples of which are discussed below.
Ventricular arrhythmias, which originate in the ventricles, include ventricular tachycardia (VT) and ventricular fibrillation (VF). Ventricular arrhythmias are often associated with rapid and/or chaotic ventricular rhythms. For example, sustained ventricular tachycardia can lead to ventricular fibrillation. In sustained ventricular tachycardia, consecutive impulses arise from the ventricles at a rate of 100 bpm or more. Such activity may degenerate further into disorganized electrical activity known as ventricular fibrillation (VF). In VF, disorganized action potentials can cause the myocardium to quiver rather than contract. Such chaotic quivering can greatly reduce the heart's pumping ability. Indeed, approximately two-thirds of all deaths from arrhythmia are caused by VF. A variety of conditions such as, but not limited to, hypoxia, ischemia, pharmacologic therapy (e.g., sympathomimetics), and asynchronous pacing may promote onset of ventricular arrhythmia.
It has been common practice for an implantable cardioverter defibrillator (ICD) to monitor heart rate, or more commonly the ventricular rate, of a patient and classify the cardiac condition of the patient based on this heart rate. For example, a tachyarrhythmia may be defined as any rate in a range above a designated threshold. This range is then divided into ventricular tachycardia and ventricular fibrillation zones. The ventricular tachycardia zone may be further divided into slow ventricular tachycardia and fast ventricular tachycardia zones.
As described above, SVTs and ventricular arrhythmias may lead to ventricular rates in excess of 100 bpm. In other words, ventricular rates of SVTs can overlap with rates of tachycardias of ventricular origin. These SVTs are often well tolerated and require no intervention. Further, physically active patients can have heart rates during exercise that overlap with their tachycardia rates. Accordingly, discrimination of VT from SVT, including increased heart rates due to exercise, may require more than just knowledge of a patient's ventricular rate. In other words, using heart rate as the sole criterion to classify the cardiac condition of a patient is often not sufficient.
To improve the specificity and accuracy of arrhythmia characterization, many implantable cardiac devices (ICDs) can also examine the morphology of an intracardiac electrogram (IEGM), in addition to the heart rate. The shape of an intracardiac complex can include information on the origin and sequence of the heart's electrical activity. A normal intracardiac complex traverses the AV node and is conducted by specialized cardiac tissue throughout the ventricles. This results in a distinctive complex morphology. A tachycardia of ventricular origin often has a different morphology due to its ectopic origin and conductance through cardiac muscle tissue. As such, in addition to monitoring heart rate, some ICDs are capable of performing morphology discrimination to classify the cardiac condition of the patient. For example, a template based on the morphology of a “known” signal can be stored in the ICD. The “known” signal can be, for example, a signal collected during a period where a patient is known to exhibit a normal sinus rhythm. By comparing the morphology characteristics (e.g., number, amplitude, sequence and/or polarity of waveform peaks, as well as the area of the peaks) of an arrhythmia to the template, the ICD can calculate the match (or lack thereof) between the waveforms. For a further description of morphology discrimination, refer to U.S. Pat. No. 5,240,009 (Williams), entitled “Medical Device with Morphology Discrimination” and to U.S. Pat. No. 5,779,645 (Olson et al.), entitled “System and Method for Waveform Morphology Comparison,” which patents are hereby incorporated by reference. These are just a few example of morphology discriminator algorithms and parameters, which are not intended to be limiting.
Sudden onset and interval stability (also know as rate stability), which are discussed in more detail below, are examples of other factors that can be monitored to improve the specificity of arrhythmia characterization. Also, the relationship between ventricular rate (V) and atrial rate (A) can be used to characterize an arrhythmia. For example, this can be part of a rate branch algorithm, which, depending on V and A, may follow one of three branches: a V<A rate branch; a V=A (within a specified tolerance) rate branch; and a V>A rate branch. If V<A, then morphology discrimination and/or interval stability may be available to distinguish VT from AF or AFI. If A and V are essentially the same (within a certain tolerance), then morphology discrimination and/or sudden onset may be available to distinguish VT from sinus tachycardia. If V>A, then an arrhythmia may be characterized as VT. Also, specific branches can be turned on or off. For example, if V is greater than the tachycardia threshold but essentially the same as A, and the V=A branch is turned off, then the algorithm can cause the V>A branch to be followed, and the arrhythmia may be classified as VT. Additional details of an exemplary rate branch algorithm are provided in U.S. Pat. No. 6,636,746 (Fain et al.), entitled “Safety Backup in Arrhythmia Discrimination Algorithm,” which is incorporated herein by reference. Also, atrioventricular association (AVA) can also be used to AFI from VT. In an exemplary (AVA) algorithm, the AV interval is measured from each ventricular sensed event to its preceding atrial event and an AVA Delta is then calculated as the difference between the second longest AV interval and the second shortest AV interval in a recent group of intervals. If the measured AVA Delta is less than a programmable AVA threshold parameter, the AV intervals are considered stable, which is indicative of SVT. If the measured AVA Delta is greater than or equal to a programmable AVA threshold parameter, the AV intervals are considered unstable, which is indicative of VT. More generally, the relative rate of the atria and ventricles and/or the timing relationship between atrial and ventricular events can be considered.
Typically an ICD is programmed to provide a therapy in response to an arrhythmia being detected, where the type of therapy corresponds to the type of arrhythmia that the ICD believes it has detected. For example, VT may be treated with a therapy consisting of low-energy pacing pulses designed to capture the ventricles. This therapy is referred to as Anti-Tachycardia Pacing therapy (ATP). VT may also be treated with relatively low energy, synchronized cardioversion shocks. VF, on the other hand, is typically treated more aggressively with high energy shocks. The ICD is programmed with numerous parameters that are used to discriminate between types of arrhythmias, and to define the types of therapies to be used to treat the various arrhythmias. An SVT may or may not be treated, depending on how an ICD is programmed.
Over the years, the number of programmable parameters has been increasing steadily. A modern ICD may have up to 200 or more programmable parameters. A major challenge for both ICD manufacturer and caregivers (e.g., physicians, clinician, or the like) is to select proper values and uses of these parameters. While the manufacturer may provide nominal values and uses of these parameters (also referred to as default parameters and criteria), these nominal values and uses may not be proper for all patients and it is up to the caregiver to change them using statistical information, knowledge of the patient and his/her condition, and the caregiver's personal experience.
Caregivers often have difficulty programming ICD parameters intended to aid in the discrimination of SVT from VT, because the implications of making adjustments to them are not clear. As a result, caregivers often either do not turn on these discriminators routinely, waiting to see if the patient will receive inappropriate therapies before attempting to program the device appropriately, or they turn the discriminators on with the default parameters provided by the manufacturer, without any attempt to customize them for the individual needs of the patient.
Historically, ICD manufacturers developed default criteria for discriminators based on the fact that the initial indications for ICD implantation included documented lethal arrhythmias or survival of an episode of sudden cardiac death. Given those indications, the defaults were chosen based on the fact that a single episode of an arrhythmia could be lethal, and therefore adhered to the philosophy that “it's better to give ten shocks too many than one too few.” In general, because these so-called “secondary prevention” patients had already survived a potentially lethal arrhythmia, they were somewhat willing to accept an inappropriate shock, because they understood the consequences of not receiving a shock when actually needed. In addition, since these patients already had documented arrhythmias and/or had undergone electrophysiology studies, the types of arrhythmias they had (e.g., VT vs. primary VF), and their hemodynamic tolerance of those arrhythmias, was often known.
Today, the ICD implant population, and the indications for implantation have changed, such that a large percentage of patients receiving ICDs or other tachyarrhythmia control devices are so-called “primary prevention” patients, i.e., those who are at risk for tachyarrhythmias and sudden cardiac death, but who have not yet shown any evidence of actually having experienced such arrhythmias. For these patients, their understanding of their need for an ICD is often much less clear, and they are, therefore, much less tolerant of inappropriate shocks from a device that they are not convinced that they need. Moreover, since these patients have not experienced arrhythmias nor undergone electrophysiology studies, the type and tolerance of arrhythmias they may experience in the future is largely unknown.
Despite the shift in patient populations, manufacturers of ICDs and other arrhythmia control devices have been reluctant to change the default values for the discriminators to something “less safe”, because not all patients fall into the primary prevention category. Coupled with caregivers' reluctance to program away from those default values because the implications/risks of doing so are unclear, the result is that the proper weighing of discrimination sensitivity vs. specificity is often not addressed by caregivers until the patient has received inappropriate therapies.
Accordingly, there is still a need to assist physicians, clinicians and other caregivers in programming discrimination algorithms in a way more appropriate to the indications and characteristics of patients.